Electricity meters are devices that perform measurements related to power and energy consumption. For example, electricity meters may be used to measure energy consumption by a customer load. Electricity meters also measure other properties relating to energy consumption such as voltage levels, current levels, and power factor by way of example. Utility service providers employ electricity meters to track customer usage for many purposes, including billing, planning and tracking demand for electrical power.
Historically, electricity meters employed electromechanical “rotating disk” meters that measured energy consumption using magnetic flux induced by line voltage and line current on core elements disposed about a rotatable disk. The rotations of the disk were registered on mechanical counters to track quantities of energy being used.
Increasingly, meters employ electronic circuits to reduce the number of moving parts, and to provide enhanced metering and data collection services. In general, electricity meters having such electronic circuits may be divided into two general parts: a sensor part and a measurement part. Meters may have additional parts, of course, such as a communication interface, radio, enhanced memory, and any number of features. However, the fundamental portions of the meter include the sensor part and the measurement part, and are present in every meter.
The sensor part includes primarily analog circuitry in the form of sensor devices that are connected to the electrical system of a facility, and more particularly, to the power lines. The sensor devices generate voltage and current measurement signals that are indicative of the voltage and current in the power lines. The measurement part includes primarily digital circuitry that receives and processes the measurement signals generated by the sensor part. As a result of this processing, the measurement part generates metering data representing, for example: watt-hours, volt-amps (“VAs”), reactive volt-amps (“VARs”) and other information that quantifies the power consumed by the facility.
Referring again to the sensor portion of the meter, in order to obtain voltage measurement signals, the sensor devices often include an electrical connection to the utility power lines that supply the load. In order to obtain current measurement signals, the sensor devices may employ a current measurement transformer or other device that indirectly obtains current measurements.
In order to facilitate measurement by electronic circuitry, the line voltage in the voltage measurement signals must typically be reduced. To this end, the measurement circuit may include step down elements, such as, for example, a voltage divider that translates line voltage values, which may be 120 Volts AC, 277 Volts AC or higher, to a low voltage measurement range of less than 5 Volts peak to peak.
Although the measurement circuit steps down the high line voltages, the high line voltages are still present within the meter in or around the sensor portion. An issue that arises with utility meters relates to these high line voltages received in the sensor devices within the electricity meter. Electricity meters sometimes encounter line surges that can exceed the level of surge protection normally provided within the meter. Normal surge protection may include elements such as fuses, MOVs, or other elements disposed on the circuit board that carries the measurement circuit. Line surges are particularly troublesome in multiphase meters because phase to phase arcing could occur in some circumstances. Phase to phase arcing in electricity meters can result in an undesirable failure mode, particularly when phase to phase voltage exceed about 350 volts.
In particular, phase to phase arcing involves a phenomenon called power follow through. In particular, phase to phase arcing results in effectively causing a short circuit between phases. The short circuit provides a low resistance over which the high phase to phase voltage must be dissipated, resulting in very large currents. The large current event that occurs as a consequence of phase to phase arcing is called power follow through. Power follow through can cause significant damage to the meter and/or the meter's surroundings.
Power follow through is particularly troublesome for phase to phase arcing as compared to phase to neutral arcs. Phase to neutral arcs present less trouble because they often extinguish themselves at the zero crossing of the AC waveform. By contrast, phase to phase arcs do not extinguish at the zero crossings, and also tend to have higher voltages to dissipate. For example, in a 277 volt four wire wye system, the phase to neutral voltage of each phase is 277 volts. However, there exists 480 volts from phase to phase. Arcs over approximately 350 volts within a meter are less likely to self-extinguish and thus can result in large power follow through. Thus, phase to phase arcing creates a greater risk of damaging power follow through than phase to neutral arcing.
The risk of arcing exists anywhere within the meter that exposed conductors of different phase voltages are present. Obviously, the closer the conductors, the greater the chance of arcing. Regardless of efforts to isolate exposed conductors carrying phase voltages, there is always a danger of phase to phase arcing for a variety of reasons.
There is a need, therefore, for a method and/or apparatus that helps reduce the potential for damaging due to line surges in electricity meters.